On the High Cost of Consciousness

Publisher Website
Google Scholar
Abstract
ON THE HIGH COST OF CONSCIOUSNESS HORTON A. JOHNSON* The human brain, deep in thought, consumes about 14 w of power. It gives some insight into world politics to consider that when three heads of state meet at the summit their combined brain power is that of a 40 w light bulb scarcely sufficient for reading the lines, to say nothing of reading between them. It is further discomforting to consider that the energy consumption of their brains is only slightly more at the conference table than when they are sound asleep. Evidently most of the 14 w represents basal metabolism, and little of this goes into conscious information processing or actual thinking. Although the energy consumption of the brain seems low by lighting standards, it is unaccountably high as compared with other organs. During sleep, the power input to the brain is twice that of the heart and only slightly less than that of the liver. And yet the brain performs no significant work on its external environment. It does neither the mechanical work of the heart nor the chemical work of the liver. Comparing energy in with useful work out, the brain is thermodynamically inefficient . But during sleep, the brain tissue is excitable. It can respond to a telephone or an alarm clock. It is in a state of consciousness; it is still turned on. The energy cost of operating a brain has to be charged to the maintenance of this responsive or conscious state. The additional cost of conscious information processing or wakeful thought is trivial by comparison . This is analogous to the operation of a radio receiver. A radio consumes about as much energy whether it is receiving a signal or not. Like the brain, it must use most of its power input not for signal processing but for the maintenance of a receptive state. Being in a responsive state requires a displacement from equilibrium, so that a small perturbation can trigger a response. The displacement from equilibrium in a radio is ?Department of Pathology, Tulane University School of Medicine, 1430 Tulane Avenue, New Orleans, Louisiana 70112.© 1980 by The University of Chicago. 0031-5982/81/2401-0209$01.00 Perspectives in Biology and Medicine ¦ Autumn 1980 113 measured by the electrical potential across its transistors. The basal or zero-signal power requirement is due to the leakage of current through the transistors and other resistances in the circuit. The analogous displacement from equilibrium in a brain is measured as electrical potential across membranes and is due to the partitioning of ions across those membranes. The basal cost of consciousness is due to the leakage ofions, chiefly sodium and potassium, across membranes. In order to maintain consciousness, these ions must be repartitioned at a cost of 14 w or 220 cal/min. There is experimental evidence (Appendix A) that in a human brain K ions leak out of the intracellular space at the rate of about 2.8 meq (milliequivalents)/min. The K ions diffuse passively back into the cells at a rate of about 1 .4 meq/min, and the remaining 1.4 meq/min are actively transported. The leakage flux of Na into the cell is greater than the outward leakage of K, with the result that in a steady state 2.1 meq/min of Na must be actively pumped out of the intracellular space. Is the active countering of this leakage necessarily a costly process? If we knew the theoretical minimum cost of reordering these ions, we could estimate the efficiency with which the brain maintains its displacement from equilibrium, its excitability or consciousness. The means of solving this problem do not lie within the domain of neurophysiology but have been set forth by four theoretical physicists: Maxwell, Boltzmann, Szilard, and Brillouin. Let us review their ideas, which are often quite whimsical, to see how each of them might have approached the theoretical minimum cost of maintaining a displacement from equilibrium in the brain. Cerebral Efficiency according to Maxwell James Clerk Maxwell, the father of electromagnetic theory, would have claimed that his demon could maintain the brain in a state displaced from equilibrium at no cost whatever. The demon was born in...

This publication has 0 references indexed in Scilit: